Nearly all electronic circuits currently in manufacture utilize printed circuit boards for the mounting and electrical interconnection of semiconductors and other circuit elements or components such as resistors, capacitors, etc. These boards are made from a stock material comprising a base of insulating material such as paper or glass epoxy having a thin sheet of copper cemented or bonded to one or both sides thereof. The base material of the board is typically from one-sixteenth to one-eighth of an inch thick with the copper runs comprising one to ten mils in thickness. The copper conductor patterns are formed by means of a photo-etching process that removes the unwanted copper from between conductor runs. After the etching process has been completed, holes are drilled through conductor pads for the mounting of the electrical components. An additional plating process usually follows to form conductive surfaces through the drilled holes. The plated-through holes permit a better and more reliable solder bond between the conductor runs and the terminals of the electrical components.
The next step in the manufacture of the printed circuit board involves the mounting of components. The individual components are mounted on the board by inserting the leads or terminals of the components through the plated-through holes. This operation is done either manually or by automated means.
Finally, these leads or terminals are soldered in place, typically by means of an automated process in which the underside of the board and the protruding leads and terminals are passed across the surface of a molten bed of solder. The molten solder is drawn up into the plated-through holes by capillary action and solidifies when the board is withdrawn from the solder bed.